1. Field of the Invention
The invention relates to a method for removing solid matter method for removing solid matter from a fibrous stock suspension by way of flotation, wherein fibrous stock suspension is mixed with gas and wherein flotation foam is formed in at least two flotation chambers and collects solid matter and removes it from flotation chamber and wherein moreover the fibrous stock suspension which has been ridded of the solid matter is discharged as through-flow from respective flotation chamber.
2. Description of the Related Art
A foam or floating sludge is formed through flotation which contains and removes matter to be eliminated. A typical application for such methods is the processing of a suspension obtained from recovered printed paper, wherein the printing ink particles are already detached from the fibers so that they can be removed through flotation. A fiber stock consistency (volume of fibers relative to the overall volume) of between 0.5% and 2%, preferably between 0.8% and 1.2%, is hereby often adjusted in the fibrous stock suspension. The flotation process described here exploits the differences between paper fiber stock and solids which are to be removed, in particular undesirable impurities in such a way that the fibrous material based on its rather hydrophilic character remains in the fibrous stock suspension, whereas the aforementioned impurity particles are hydrophobic and therefore migrate into the foam together with the air bubbles. Therefore, not all solid matters are removed through flotation; however, fibers are separated from contaminants. As a rule, the often used term “flotation deinking” is not only used to remove printing ink particles, but also more generally for the flotation of fines from the fibrous stock suspension. Besides the aforementioned printing inks, additional contaminants include in particular stickies, fine synthetic particles and possibly also resins. Controlled removal through flotation of mineral filler materials (“ash”) may also be one of the objectives of the method.
The current state of the art in regard to flotation methods for fibrous stock suspensions is already very advanced. Solutions are available which are definitely suitable for removal of solid particles through flotation in the desired method and volume. However, good flotation results are obtained with a relatively high expenditure, especially in regard to equipment, operating resources and energy. The problem is in meeting two requirements, namely on the one hand complete removal of all materials which are to be rejected, as a rule removal of the impurities, and on the other hand avoiding losses, namely the unintended removal of materials which are to be utilized for the product to be produced at a later stage. In the practice of flotation these two objectives can be met equally well only with complex procedures.
In order to achieve optimum separation of floatable materials as well as only a very small loss, multi-stage equipment can be used. The fibrous stock suspension hereby flows usually consecutively through several flotation chambers or flotation cells assigned to one flotation stage until the desired material removal is achieved in the through-flow, or in other words in the accepted stock. Since the overflow, namely the flotation foam, which is formed in this flotation stage still contains a considerable amount of for example paper fibers it is directed as inflow into a further stage. Normally one refers to a first and second flotation stage, or also to primary or secondary flotation. The through-flow, namely the accepted stock of the second flotation stage can again be added to the incoming flow of the first flotation stage. There are also cases in which the through-flow of the second stage is mixed to the through-flow, namely the accepted stock of the first stage. The flotation foam produced in the second stage can then be disposed of or if it still contains too many fibers can be directed to a third stage. Flotation chambers or cells for the second flotation stage are usually designed similar or identical to those for the first stage, however there are substantially fewer in number. Typical installations have five or six flotation cells in the primary stage and one or two in the secondary stage.
DE 101 25 978 C1 describes a two-stage flotation unit whereby partial flows are drawn off and re-added at another stage in the unit. In particular, flotation foam is returned into the fibrous stock suspension of flotation cells located upstream.
It is the objective of the current invention, and what is needed in the art is, to reduce the expenditure associated with implementation of the method without occurrence of losses in regard to the separation efficiency.